I. Field of the Invention
The present invention relates to a control method for antilock braking systems capable of precluding the locking of the wheels of a motor vehicle on sudden application of the brake and to a measurement device of the vehicle wheel operating force capable of using antilock braking systems.
II. Prior Art
If the wheels of a motor vehicle are locked upon application of the brake of the motor vehicle, rotation of the affected wheels is prevented followed by tire slipping on the road surface. Such a condition means less vehicle controllability.
Antilock braking systems (ABS) are control systems capable of preventing the wheel from locking during the braking process. Generally, a conventional antilock braking systems for motor vehicles employ a system such that the braking action is automatically controlled in accordance with the speed of the vehicle wheel. The vehicle wheel speed is measured using a wheel speed sensor. This wheel speed sensor has structure which measures rotational velocity of the vehicle wheel by detecting the speed of rotation of a geared hub attached on the wheel axle with the use of a magnetic pick-up.
The control procedure of the conventional antilock braking system is as follows: when braking is suddenly applied, the locking of wheel is detected by the rapid decreases of the wheel speed. The control system responds by reducing braking pressure. Thereafter, when the wheel speed recovers due to the reduction in braking pressure, braking is again applied. In such a control method, however, the effect of brake control is lost in fact during a period from the occurrence of the wheel locking to the resumption of the braking process. Consequently controllability becomes unstable and it results in a longer stopping distance for the vehicle.
In one conventional antilock braking system, control of the system is commenced by using an arbitrarily chosen control method and parameter settings. Thereafter the slipping frequency is counted during control and the friction coefficient of the road surface on the basis of the counted slip frequency is estimated. When the slip frequently occurs, the control method is changed to a method suitable for the road surface having a lower friction coefficient, and when there is no slip, it is switched to a method suitable for the road surface having a higher friction coefficient. However, such a control method causes an increase in the loss of the breaking force during tire slipping and results in a longer stopping distance for the vehicle.
It is possible to control the operation of antilock braking systems by using a measured value of a stress applied on the vehicle wheel. There are known a photoelasticity method; a brittle coat method; a holography method; a strain gauge method, etc. as methods measuring stress about a structure. Generally, the strain gauge method is widely used, but it is difficult to detect an intended stress value because it receives forces other than the stress to be measured. In particular, there is no method detecting side forces.